box port measurments help

[ncc74656 2]

so in torres i put a 23.5" length into the calcs. this gives me a 35.07hz tune with a port dimension of 23.5X4.75" and 7.07 cubic. the physical port length is 17.25 according to torres.  i need to verify what my port actually is and i feel a bit confused by the measuring.  if i measure the port down teh center wich is how i understand the correct measurement to me then i get 20 and 3/8" in length, then i measure horizontally to the inside of the MDF sheet wich is 3 and 1/8". so that makes the port 23.5" long down the center. if i measure on the face of the wood itself i get  18.75" long and accounting for 3/4" thickness of the port itself. on top of that it makes it 2.375" longer on all accounts if i take 1/2 of the port width for "end correction".  what measurement is the correct way to express the desired length?

 

the baffles are 1.5" thick and all other wood is .75" thick.

 

the ultimate question is: do i need to add or subtract anything from this port to make a 35hz tune?

 

here are pictures: in the picture the horizontal length does not take itno account the 1.5" thick baffel 

 

http://imgur.com/a/DTYGB

Edited June 1, 2014 by ncc74656

[frogcase2002 393]

So there is no bend in the port? Don't think you will get the tuning you want when the enclosure is as deep as your wanting the port length

[///M5 2,833]

I don't understand your question. Measure down the middle - yes.

Does your shop not ever build a ported box? wth?

[frogcase2002 393]

I don't understand your question. Measure down the middle - yes.Does your shop not ever build a ported box? wth?

Most shops iv seen don't. A lot of shops "custom enclosures" iv seen is fiberglassing over a prefabricated enclosure to make it look fancy or be a custom fit in a vehicle

[///M5 2,833]

Confusing since it only requires 5th grade math. I am somewhat not surprised locally though as I've never been in a shop in MN where they knew ANYTHING.

[ncc74656 2]

i have never heard of a shop simply covering up a prebuilt box but in either case myt question is this:

 

so I measure down the center of the port and if the port is the same distance from the back of the box as it is wide then 1/2 of the port width is where the measurement stops. then from there you measure from that point out horizontally until you reach the end of the internal port. so in this instance I have a 3.125" length from this center port to the inside edge of the port as it does not have any extension on it. now if I subtract say 1" from this port it will then be 5.75" from the back wall which would then make the port 17" long plus a 2.375" port correction. thus removing 4.125" of port by removing 1" of physical port.

 

is this correct?

[ncc74656 2]

all things being equal if the "port" that the woffer sees is the correct length factoring in end correction would it matter if the port bends or not? all else being equal if the box were say 5" shorter and thus the port bent would that have any effect on the tuning?

 

 

edit for context, this is the guide im using to measure port length: it wont let me past the URL but its a link to "8 basic diagrams to measure port length"

Edited June 2, 2014 by ncc74656

[frogcase2002 393]

Yea I still don't understand what's being asked.... seems like you should have someone design the enclosure for you..................just measure the port size you need.don't try to keep making things work. The port length is the port length

[frogcase2002 393]

I don't think your making the port as long as you want it.... and thinking of ways for it to seem longer

[altoncustomtech 773]

I think what the OP is getting at is that his port runs right up to the back of the enclosure.  At the current length he's long when measured down the center of the port, including the bend the air has to make at the back of the enclosure which is part of the path.  I think what his problem is that if he reduces the port length an inch to theoretically take bring it from the back and take the bend out of the equation that he's coming up too short.

 

Is that close to the issue?

[bulldogfreddy 2]

This is what I came up with so far if it helps. I got 6 1/4" down the center in the rear. So you can add 20 3/8" + 6 1/4" then you can take you port correction from there. Hope the picture helps and is close to what you had.

Edited June 2, 2014 by bulldogfreddy

[ncc74656 2]

yea, I think that's close to it. the total length by 12 volt port calculator is 26.3" of port required. once im done with work ill take a closer look at the pictures. I just want to make sure I hit the frequency im aiming for. having the box depth at just the right amount to make the port bend/calculations some what dicey for me... I may just shorten the box, widen it and make it easier on the computations.

[altoncustomtech 773]

Use the calculator to calculate what the tuning would be with the different length scenarios you're talking about.  You're not really going to hear any difference if the tuning is off one hertz or so.  I mean, unless it's a competition box that you're trying to nail the tuning to one exact frequency to match cabin gain or something to get every single dB out of it you can it's not going to hurt anything to be just a tad off.

 

Ears aren't going to be able to tell the difference between a 31hz tune and a 30hz tune, especially at high output levels.  As long as it's close just go with it.

 

It's also possible, especially if space allows, to plan the enclosure a little on the large side with slightly lower than desired tuning, then just add bracing and/or filler blocks to get the desired airspace/tuning.  FWIW